It is known that glass may be strengthened by ion exchange. In one process, small ions on the surface layers of glass are replaced by ions of larger size having the same electric charge. Thus, by way of illustration, sodium ions in the surface of an article made of soda-lime glass may be exchanged for potassium ions, which are larger, thereby creating compressive stresses in the surface layers and putting the inner layers under tension. In this process, to facilitate the exchange, the glass is heated; but the temperature should be kept below the strain point (the point at which the particular glass has a dynamic viscosity of 10.sup.14.5 poises).
U.S. Pat. No. 3,486,995 of Evers discloses that, in general, the ion exchange takes place by the laws of diffusion. Evers states that: "The exchange takes place when, at a given temperature, the exponential function of the rate of movement of the ions to be exchanged coincides with their constant of diffusion, but with the temperature below the transformation point...the speed of thermal diffusion is slight, and the exchange of ions progresses slowly"(see lines 49-57 of column 1).
In the conventional ion exchange processes, unless the ion exchange is allowed to progress for an inordinate amount of time, the depth of penetration of the ions will be relatively shallow. Such shallow depth of penetration causes several problems. Thus, as is disclosed in U.S. Pat. No. 3,293,016 of Cornelissen et al., the strenghtening effect obtained with shallow depth of ion exchange is not long-lived. Cornelissen states that "Generally the resulting strengthening is not permanent in normal use of the articles. By damaging the surface the obtained strengthening is largely cancelled. It has now been found that in this case too thin a surface layer has been produced which lost its effect by the damage normally done in practical use"(see lines 25-30 of column 1).
The depth of penetration can be increased by extending the time of treatment. However, as is stated in U.S. Pat. No. 3,751,238 of Grego et al., "...this expedient also greatly increases the tension in the central zone of the article. In the extreme case, this results in total disintegration of an article with explosive force when breakage occurs. Such a situation is normally highly undesirable, and a variety of methods have been suggested for its alleviation"(see from line 66 of column 1 to line 3 of column 2).
In addition to causing the problem of explosive breakage, use of a prolonged treatment time is relatively expensive. Thus, as is disclosed on page 5 of D.A. Copson's "Microwave Heating"(The Avi Publishing Company, Inc., Westport, Conn., 1962), "In the application to electronic heating, one can express the energy needed as, 2. energy=power.times.time
where energy is defined in watt-hours, power in watts, and time in hours...."
Furthermore, long treatment times, in addition to requiring substantial amounts of energy, adversely affect productivity in large-scale commercial facilities.
With conventional ion exchange processes, suitable depths of ion exchange require hours. Thus, for example, in the ion exchange of potassium ions for sodium ions, treatment times of about 24 hours or more are usually required for the potassium ions to penetrate to a depth of from about 70 to about 100 microns.
It is an object of this invention to provide an ion exchange process for strengthening glass which, for a specified treatment time, provides substantially greater depth of ion penetration than prior art ion exchange processes.
It is another object of this invention to provide an ion exchange process for strengthening glass which, for a specified depth of ion penetration, requires substantially less time than prior art ion exchange processes.